wl1271: Check interface state in op_* functions

Check the state of the interface on op_* function so we don't try to access
the hardware in when its off.

The mac80211 may call these in some corner cases related, for instance, to
the hardware recovery procedure. These accesses cause a kernel crash on at
least some SDIO devices, because the bus is not properly claimed in that
scenario.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>